Thickness tolerances cannot be avoided during the production of plastic sheets from plastic film by film blowing. These tolerances arise, for example, because of the customary manufacturing tolerances in the blowing head of the extruder used for the production of the film, and in the sizing unit through which the tubular sheet passes with an expanded circular cross section after the extrusion process, because of the internal and external cooling or due to other external circumstances. When such tubular sheets are rolled up in the collapsed state in wind-up equipment, the addition of the thickness tolerances would result in annular elevations, if there were no suitable countermeasures on the winder. These elevations would result in a lasting deformation of the film in this region and would make the printing and further processing of the tubular sheet difficult.
As a suitable countermeasure for preventing the formation of annular elevations on the film winder, so-called reversing take-off devices for the tubular sheet have been developed, in the case of which, during the collapsing of the tubular sheet, the reversing motion of the participating parts of the apparatus bring about a periodic migration of the folded edges, which are formed during the collapsing of the tubular sheet. Associated with this is a corresponding distribution of possible thickness tolerances over the width of the collapsed tubular sheet. During the subsequent winding up of the collapsed tubular sheet or of the individual sheets, previously severed in half, on wind-up equipment, the formation of annular elevations on the reeled film by the addition of thickness tolerances of the film is avoided in this manner. It is self-evident that it is necessary, at the same time, to select a swiveling angle of sufficient magnitude for the reversing motion.
In the case of a known apparatus (DE-C 20 35 584), the collapsing boards with their nip rolls are swiveled in a reversing manner over an angle of about 360.degree. about the axis of the tubular sheet supplied and it has turned out that this swiveling angle of a reversing motion is appropriate for most of the applications of reversing take-off equipment. Aside from two rotatable deflecting rollers for deflecting the sheet by about 180.degree., two turning bars, which cannot be rotated, for deflecting the sheet by about 180.degree. while simultaneously changing the direction, are provided one behind the other and alternating with one another. On the other, in those cases, in which the swiveling angle of the reversing motion of the collapsing boards with their nip rolls is less than 360.degree., a single turning bar between the two deflecting rollers is sufficient for handling the tubular sheet in the reversing part of the take-off apparatus. However, it is also possible to use more than two deflecting rollers and turning bars.
In the case of the known take-off apparatus, the deflecting rollers and the turning bars in the apparatus are supported pivotably about the axis of the tubular sheet supplied in such a manner, that the axes of the deflecting rollers and of the turning bars extend tangentially to the circles about the axis of rotation of the collapsing boards, the deflecting rollers, in all possible operating positions, being radially outside of the parts of the turning bars looped by the tubular sheet. The swiveling angles of the deflecting rollers and of the turning bars decrease as the distance from the collapsing boards increases and the film bubble is passed between the deflecting rollers and turning bars, as well as the stationary conveying equipment for transporting away the collapsed tubular sheet in mutually parallel, horizontal planes. By these means, a small overall height of the reversing system is attained. Moreover, due to the fact that the radius of the circle, circumscribed by the swiveling path of the axes of the turning bars, is .pi./4.times.diameter of the turning bar, a lateral course of the tubular, plastic film sheet during the swiveling of the reversing system is counteracted. At the same time, it turns out that the intersection of the center lines of the tubular sheet extrudates, running towards and away from the turning bars, lies in the central swiveling axis of the reversing system. During the reversing swiveling motion, it is thus ensured that the tubular film extrudates open and close in scissor fashion about the swiveling axis, so that theoretically mutually opposite, identical friction forces act symmetrically on the two tubular sheet extrudates and thus, by design, the axial shifting of the tubular sheet on the turning bars, during the reversing operation resulting from unilaterally acting frictional forces, is avoided.
In the case of the known take-off apparatus, the collapsing boards, the deflecting rollers and the turning bars, for carrying out their reversing pivoting motions, are all supported at a relatively thin middle shaft, which extends in a vertical extension of the axis of the supplied tubular sheet and is supported rotatably in the apparatus frame. The total weight of the reversing system is supported by the middle shaft. As a result, the reversing system is very susceptible to fluctuations and to oscillating motions which, especially due to the long vertical take-off path corresponding to the height, at which the take-off equipment is disposed, set in very quickly when, for example, an operator moves on the associated gangway of the take-off equipment. Moreover, it is time-consuming and cumbersome to install and maintain the known reversing system and, in particular, also to introduce the tubular sheet into the take-off apparatus at the start of an extrusion process because of the supporting shaft, which is disposed in the center of the equipment in an extension of the axis of the tubular sheet supplied, because this central region is not readily accessible and occupied partly by the tubular sheet, which is being conveyed through the apparatus.
The EP-B-0 408 996 describes a known take-off apparatus with several turntables which are disposed vertically above one another and can be driven in a reversing manner over a reversing transmission with different swiveling angles. The frame for the equipment comprises an upper, horizontal support part with a holding frame for supporting the turntables on the underside so that they can rotate and, for forming a set of compression-stressed turntables rotatable about the axis of the tubular sheet, the turntables and holding frames are supported directly on one another or on the holding frame over interposed roll bodies. For this known configuration, the internal diameter of the turntables is selected to be equal to the maximum working width of the equipment, which corresponds to the largest possible sheet width of the collapsed tubular hose that can be handled in the equipment. On the basis of these dimensional relationships, the set of turntables represents a stable structure, by means of which interfering fluctuations and oscillating motions are practically excluded when anyone is walking on the gangways or working platforms mounted on the frame of the equipment. On the other hand, however, the accessibility of the reversing system from the direction of the collapsing boards is impeded by the interposed set of turntables with its holding frame.